As Samuel is driving you to your drop off point, he talks about Tall Boys, which are guards using big walking machines that pack a serious punch. Go up the stairs and you'll be rather close to a Tallboy, so don't get any closer. You want to get into the Boyle Estate compound, which you can do by either going over the gate or sneaking through a small tunnel just to the right of the gate.
Once you're inside, put away your weapons (hold <Action Button>), as you will want to blend in, and having your weapons out isn't exactly blending in. Approach the front door and the one lady's invitation will fly away, so go and grab it, then present it to the doorman. You will now be allowed inside.
(OPTIONAL) DUEL LORD SHAW
Before you left The Hound Pits, Pendleton gave you something to give to Lord Shaw. When you're inside the compound of the Boyle Estate, make a right and go and talk to Lord Shaw. It turns out that Pendleton was supposed to duel Shaw, but you're now taking his place. You don't have to kill, as you can switch to a Sleep Dart and shoot him with that to complete the duel. Grab the gun, go to the circle marked, and switch to the Sleep Darts. On the count of two, start turning around and fire when you're supposed to. If you get shot, you will die.
FINDING THE REAL LADY BOYLE
This mission is random, in the fact that there are three Lady Boyles and you don't know which one is your target. In fact, every time you play this mission, your target will change, so you will need to find out every single time. There are two ways to find out which one is the real target, which you gain clues to by talking to guests. One is by going upstairs and searching the bedrooms for clues, and the other is by talking to Miss White (moth mask) and doing her little side mission, which will reward you with the real identity.
For the nonlethal method in this mission, you will need to find and talk to Lord Brisby (rat-like or something mask), who will bring you aside and say he will take care of your target if you bring her to the cellar. For now, talk to Miss White, who wants a drink. Go get a drink from the Cide Fountain and bring it back to her and as your reward, she will tell you who your target is.
ELIMINATING LADY BOYLE
Go and talk to the real Lady Boyle, whichever one she is. There is an easy way to get the Lady to the cellar, without doing much more than talking to her. When you're talking to her, say she's in trouble tonight and that you can help her, eventually steering her in the direction that she should go to the basement. Follow her, until she finally goes into the kitchen and then into the basement, then choke her out while she's down here.
When you have Lady Boyle unconscious, there's a switch near the locked door that leads into the basement. Open the door using the switch, then bring her to Lord Brisby in the boat. Once he takes her, there's a door right around the corner that leads into the sewers. Use this to get to a gate that can be opened with a lever, then swim your way (careful of the Hagfish) to Samuel's boat to finish the mission.
